The invention relates to a method for mending damaged areas in papermachine fabrics made form synthetic resin woven fabrics, especially sheet forming fabrics and drier fabrics woven entirely or partially from polyester monofilament.
While the service life of phosphor bronze papermachine fabrics used nearly exclusively up to 1968 lasted several days or weeks, the service life of synthetic resin papermachine fabrics is several times as long. The price of synthetic resin papermachine fabrics is substantially higher than that of phosphor bronze fabrics. Any damage occurring to synthetic resin papermachine fabrics during operation constitutes a considerable economic disadvantage. Therefore, various methods have been developed to mend damaged areas in synthetic resin papermachine fabrics. In case of larger tears the papermachine fabric cannot be saved because the force transmitting ability of the fabric is interrupted. Sheet forming fabrics transmit force in the range of 120 to 150 kW/m of wire width. In areas where the longitudinal wires are interrupted over a major width the fabric forms a longitudinal wave or so called ridge so that the fabric becomes useless. In case of minor damage areas having a diameter of less than 10 mm, which occur more frequently, there is generally a desire in the papermaking industry to continue using the papermachine fabric at least for some time. Attempts must therefore be made to mend such damaged areas.
Simple stitching over the hole of such a damaged area with synthetic resin or metal filaments is ineffective as such filaments are soon worn through. Another possibility of mending such damaged areas is invisible mending. To this end individual wires around the damaged area are removed from the fabric and are replaced by substitute wires from a reserve patch of the same fabric which consequently have the same knuckles as the wires of the defective papermachine fabric. The substitute wires are threaded in longitudinal and in transverse direction into the papermachine fabric at the defective area and are so interlaced that the original fabric structure of the damaged area is restored. Such a mending method results in a durable repair. However, it is very difficult to perform, as it can be carried out only by trained and experienced personnel. Moreover, said mending method takes a long time so that it can be used only in rare cases. Normally the machine shut down costs very soon exceed the value of the papermachine fabric.
Moreover, methods have been known by which the damaged area is patched up with a woven patch. The fabric selected for the patch must be rather thin so that it does not form too big a hump on the paper face of the papermachine fabric and does not leave excessively deep marks or even holes in the paper web. Monofilamentary woven fabrics, resin impregnated multifilamentary woven fabrics and open leno weave fabrics are employed for the patches. The adhesive is selected among commercial synthetic resins, hot melts, and two component adhesives. The durability of such mended areas varies. Difficulties are encountered particularly in choosing the proper amount of adhesive, because sufficient adhesion of the patch fabric must be attained and, on the other hand, a minimum of interstices in the paper machine fabric should be clogged in the area of the damage. An advance has been achieved with the use of thermoplastic adhesives. In a pretreating step the patch fabric is impregnated with thermoplastic resin. The patch is then fixed to the damaged area by simple ironing in the papermaking machine. However, in this case too, the mending will not last longer than a few days. The patch then comes off in most cases and must be renewed.
Attempts have also been made to employ perforated adhesive film or adhesive nonwoven fabric. Also, with this type of patch the clogging of fine openings in the papermachine fabric could not be prevented.
With some success, damaged areas in papermachine fabrics have been mended by so called ultrasonic welding. To this end, a patch of polyester fabric is welded onto the defective area whose size is generally substantially larger than the damaged area itself and has about two to three times the diameter of the defective area. In general, the patch is welded to the fabric along its margin. This method requires suitable equipment for a durable repair, especially an ultrasonic welding machine, and long experience. Since both the patch material and the material of the papermachine fabric are fused together along the weld seam, the exact metering of applied ultrasonic energy is prerequisite for long durability of the repair. If too little ultrasonic energy is applied the patch will come off again. If too much ultrasonic energy is applied the papermachine fabric will be damaged, thereby increasing the damaged area. Moreover, in welding polyester a general problem arises in that polyester turns brittle when heated to high temperatures. Consequently, the mended damaged area has only low tenacity. Therefore, the chief problem in this mending method is the proper selection of the required level of ultrasonic energy.
Ultrasonic welding is applicable only to single layer fabrics. In the case of double layer fabrics or in the case of fabrics having two or more layers, damaged areas cannot be mended by applying a patch by means of ultrasound. The reason probably is that the ultrasonic pulses are absorbed, by the plurality of layers of weft and warp threads or by the plurality of fabric layers, to such a degree that a durably weld cannot be achieved. If the contacting pressure is increased, the fabric immediately melts away. The required precise selection of the contacting pressure and the ultrasonic energy is therefore generally not possible with double layer or multilayer fabrics in practice. However, in view of the higher expense of double layer or multilayer papermachine fabrics, there is a considerable interest in a practical method for mending damaged areas in such fabrics.
A further difficulty resides in the fact that the patch fabric must be rather thin so that it will protrude only minimally above the wire surface and will mark the paper web in the least possible way. However, thin patch fabrics, especially when the damaged area is rather large, are drawn by the vacuum of the suction boxes toward the running side and are worn through very soon. If the damage extends over a larger area it is therefore necessary, within the damaged area, to adhere an additional piece of fabric from the bottom side to the patch fabic in order to improve the wear resistance of the mended area.
Two wire formers are frequently made such that only one wire is driven, while the second wire is carried along by friction between the wires. Such interwire friction soon peels off any patches applied to the surface of one of the wires. The same may occur in normal papermachines when the first external wire roll has an auxiliary drive, and said wire roll rotates faster than the papermachine wire, which happens time and again in practice. This wire roll abrades the top side of the papermachine wire and any patches adhered to the top side are instantly worn through or torn off.